Optical fiber splices and connectors, collectively designated hereinafter as "connectors," are necessary components of any optical fiber system. Inasmuch as optical fibers have finite lengths, it is necessary that a connector be able to splice the ends of two fibers together. Also, where a break occurs, a device, such as a connector, must be able to join the pieces of the broken fiber. In addition, in virtually all optical fiber transmission systems, the fiber or fibers terminate at either active or passive devices to which they must be connected by suitable connections. Such connections also make it possible to rearrange or reroute transmission paths and usually comprise one or more connector assemblies or sub-assemblies having some type of clamping or holding mechanism.
Regardless of the nature of the connection, the principal criteria for establishing such connection are that the connector must join two fiber ends with minimum insertion loss and that it must be mechanically stable in the working environment. Low insertion loss depends on fiber alignment, the optical flatness of the ends of the fibers to be joined, and a minimum gap between the fiber ends. Stability depends primarily upon the mechanical design of the connector, with particular emphasis upon minimizing thermal effects. In those connectors where the fibers being joined abut, the mechanical design must maintain the abutting relationship despite application of external forces.
In many optical connectors, a few millimeters of each end of the optical fiber is bonded within a coaxial bore of a ferrule. The end of each ferrule is polished to expose the optical fiber and the ends of two ferrules are forced against each other to couple the ends of the two fibers. To mate the ends of the ferrules and to maintain a proper alignment between the ends of the ferrules, the ferrules are typically inserted within an alignment sleeve.
The diameters of the ferrules are very small and may differ by several microns. An alignment sleeve must therefore be able to receive a range of ferrule diameters yet still conform as close as possible to the diameter of a ferrule. Additionally, an alignment sleeve must be able to conform to the size of the ferrule's diameter without causing an offset in transverse alignment. As a result, the opening of an alignment sleeve must be able to expand in order to accept a ferrule having a diameter which is larger than the opening of the alignment sleeve. In addition to being compliant, an alignment sleeve must be longitudinally rigid, which means that the alignment sleeve must be rigid enough to withstand forces that tend to move the ferrules out of alignment.
One type of alignment sleeve, which is rarely used, is a closed alignment sleeve of a cylindrical shape with an inside surface having a circular cross-section. To accommodate ferrules of different sizes, the walls of the sleeve must stretch, which, for most materials, requires a considerable force. While such a relatively rigid sleeve will provide nominally uniform contact between the sleeve and the ferrule, the sleeve cannot accept a ferrule with a diameter much larger than the sleeve. Because the closed sleeve does not easily conform to the shape of the ferrule, the closed alignment sleeve must be manufactured with very tight tolerances in order to have a satisfactory sleeve to wall interaction. As a result, the satisfactory closed sleeve of this type is difficult to manufacture and is relatively expensive.
Because of the difficulties and expenses in achieving these very tight tolerances, optical connectors frequently employ an open alignment sleeve, which has a slit formed along the length of the sleeve. The open sleeve expands to accommodate a ferrule by a bending action in the walls. Since the walls of the sleeve can bend much easier than stretch, the open sleeve is a much less rigid structure and consequently, has much less stringent manufacturing tolerances than the closed sleeve.
A typical open sleeve has a circularly shaped inside surface. For an example of an open sleeve, see, for instance, U.S. Pat. No. 4,850,670 to Mathis et al. In general, an open sleeve does not provide uniform contact with the ferrules. Instead, the contact between the ferrules and the open sleeve is isolated in the region near the slit and in a region directly opposite the slit. In all other areas of the open sleeve, the open sleeve usually does not contact the ferrules. For a more detailed discussion on the interference between an open sleeve and a ferrule, see, for instance, W. W. King, "Interference of a Uniform Open Ring With a Rigid Cylinder," Journal of Applied Mechanics, Sep. 1989, Vol. 56, pp. 717-719. The open sleeve therefore provides incomplete ferrule to sleeve contact.
The open sleeve has been modified to provide for more contact around the circumference of the sleeve. For instance, U.S. Pat. No. 4,541,685 to Anderson discloses an open sleeve in which the thickness of the sleeve's wall varies along the circumference of the sleeve to provide more uniform contact between the sleeve and the ferrules. This type of sleeve, however, is necessarily larger than a uniform thickness sleeve and is not suitable for many optical connectors.
A commonly used open sleeve is a ceramic sleeve having three lands spaced approximately 120.degree. apart. The behavior of this sleeve, however, is similar to that of a typical open sleeve. Consequently, this sleeve is asymmetrical and structurally biased.
Other types of devices for aligning the ferrules are also known. For instance, U.S. Pat. No. 4,050,781 to Beauhaire, U.S. Pat. No. 4,545,644 to DeVeau, Jr. et al., U.S. Pat. No. 5,220,630 to DeVeau, Jr. et al., and U.S. Pat. No. 4,880,291 to Aberson, Jr. et al. all disclose a generally triangularly shaped clip in which the ends of the fibers are inserted between three alignment rods. This type of alignment device is advantageous in that it provides three lines of contact with the ends of the fibers. Also, U.S. Pat. No. 4,691,986 to Aberson, Jr. et al. discloses a variety of alignment sleeves, including rigid and compliant shell continuous sleeves and corrugated sleeves.
Another type of open sleeve is disclosed in U.S. Pat. No. 4,205,898 to Matthews et al. This type of sleeve is formed from tubular spring metal and has a delta-shaped cross-section. One side of the sleeve has a pair of spring flaps which move outwardly to accept an inserted ferrule. Because this is an open sleeve, this sleeve is asymmetric and has a structural bias.
Many of these alignment devices, however, are too large for many of the existing optical connectors, such as an SC connector. Thus, a need exists for an alignment sleeve which may fit within existing optical connectors, have less structural bias, and be easily and inexpensively manufactured.